Plant Pathogens and Biocontrol Agents

Plant Pests

Pathogens Predators Weeds

Symptoms of Microbial Diseases in Plants

Necrosis - death of plant cells; may appear as spots in localized areas

Canker - localized necrosis resulting in lesion, usually on stem Wilt - droopiness due to loss of turgor Blight - Loss of foliage Chlorosis - loss of photosynthetic capability due to bleaching

of chlorophyll Hypoplasia - stunted growth Hyperplasia - excessive growth Gall - tumor

Pathogens : Viruses

Transmission of viruses Insect vectors - especially aphids, whiteflies,

leafhoppers, mealybugs, ants Nematodes Seeds from infected parent plants

Airborne transmission Infected plant parts Aphids Pollen

Pathogens: Bacteria Generally Gram-negative bacilli: species of

Erwinia, Pseudomonas, Xanthomonas, Agrobacterium, and Corynebacterium

Dispersal from plant generally passive by water, wind-blown water, animals, agricultural workers

In warm, humid climates, where dew and rain are common, dispersal of bacteria by rain-splash is the major means of disease spread

Airborne spread on rafts of plant material

Pathogens: Fungi Over 70% of all major crop diseases are

caused by fungi Thousands of fungal species recognized as

plant pathogens Fungal diseases cost more than $3.5 billion to

US farmers alone In general spores of most fungal pathogens

are adapted for airborne transport

Plant Pests and Their Control by Fungi and Bacteria

Plant Pathogenic Nematodes

Obligate parasites Feed on roots of plants Some never live in soil, they survive in host

and are spread by insect vectors Reduces crop yield and increases risk of

infection through wounds

Insects

Damage or destroy crops before and after harvest

Larval stage often most destructive Injury plants directly by using plant for food

or shelter and indirectly by spreading pathogens

Basic feeding patterns

Chewing Insects Either larvae or adults Tear or bite portions of the plants May eat their way through the plant causing holes and

tunnels Others eat whole plant

Sucking Insects Pierce the plant and sucks up the sap Results in curling, stunting, deformed parts

Weeds

“Unloved plant” Injurious to agricultural crops Loss is a direct result of competition for

light, water, nutrients

Control Measures

Chemicals widely used to control plant pests and diseases

Dangers of pesticide use apparent

Integrated Pest Management (IPM)

Multifaceted approach to disease control Sanitation Crop rotation Genetic resistance Biological Control

Biological Control Use of living organisms to reduce disease due to

competition or antagonism i.e.. ladybugs to control aphids

The aim is to reduce dependence on chemicals Today emphasis on microorganisms

Bacillus thuringiensis for insect control Several Pseudomonas species for control of bacterial

and fungal pathogens Numerous fungi for insects, nematodes, fungal

pathogens

Bacillus thuringiensis

Common soil bacterium well known for its ability to produce crystalline proteins with insecticidal properties

Since 1960s Bt available as a safe naturally occurring biopesticide Use as a dried inoculum containing endospores and

crystals of insecticidal proteins used as sprays or dusts for a wide variety of insects -

especially Lepidopteran

Bt Toxins

Toxins activated by enzymes in insect gut Kill insects by binding to membranes in

digestive system and creating pores in membrane~contents leak into body cavity

Harmless to humans, natural enemies of arthropods, and non-target organisms

Bacillus thuringiensis

B.t. subspecies kurstaki is widely used in caterpillar control in agriculture and forestry

B.t. subspecies israelensis is active against mosquitoes and black flies

B.t. subspecies tenebrionis is active again beetle larvae

Bt Uses

Spray ApplicationsBt toxins degrade within a few daysEndospores can survive for several years

after spray applications Genetic Engineering with Bt genes

Transfer into crop plantsTransfer other bacteria

Pseudomonas species

Pseudomonas fluorescens for control of fire blight (also may control apple blue mold)

Fire blight – bacterial disease of apples and pears caused by Erwinia amylovora

Pseudomonas out competes Erwinia Reduces use of streptomycin which has been

helpful since many Erwinia strains resistant